1. Technical Field
The present invention relates to communication systems employing time division multiplexing (TDM).
The invention is more particulary concerned with radio-relay communications systems wherein messages are transmitted to their destination via at least one intermediate radio station.
2. Prior Art
Previously proposed radio-relay systems employing time division multiplexing have employed a number of radio stations each with a radio receiver and transmitter, a multiplexer and demultiplexer for communication with each other station. Thus an intermediate radio-relay station which has to communicate with two other stations has two transmitters, receivers etc., one set for each station with which it can communicate. Each user who is connected to a station has a terminal which is connected to an input port of a transmitter multiplexer and a port of the corresponding receiver demultiplexer. The terminals may be connected via land lines or preferably via radio links. The radio transmitters of a radio station each transmit a time division multiplexed signal which may include, for example, 30 channels from 30 different user terminals. If the radio station is an intermediate radio-relay station, signals in at least some of the channels will normally need to be re-transmitted. Therefore output ports of the receiver demultiplexer corresponding to such channels are each connected to a port of the other transmitter multiplexer. Any user terminals connected to such an intermediate radio-relay station will be connected to the remaining ports of either receiver demultiplexer and corresponding transmitter multiplexer.
In such a radio-relay communication system, it is necessary to provide a pair of multiplexers and demultiplexers at each intermediate station.